Open a sprite's texture or shader from the menus and inspector (#221)

A sprite's texture and shader were only reachable by hunting for them
in the asset panel. Every place that shows such a reference now leads
straight to it.

- Object menu (hierarchy and scene view): "Open texture" and "Open
  shader" sit right below "Open asset" when the sprite has one. The
  object's own values count, so an overridden shader opens the
  override.
- Asset context menu: the same entries at the top.
- Inspector: every asset field with a value shows its label as a blue,
  underlined link. One click selects the asset, a double click opens
  it. An override stays gold and gets the underline too.
- The first click rebuilds the inspector, so the second one lands on a
  different row. The inspector catches that second click before any
  row sees it.
- New pure rule `references.ts`: the asset behind a reference field. A
  frame leads to its sheet. Kinds without a tab (palette, font, sound)
  get no menu entry.
- `followsPress` in `clicks.ts` keeps the double-click window and
  tolerance in one place.
- `activateAsset` in `actions.ts`: opening means the same everywhere
  as a double click in the asset panel, so a scene also becomes the
  active one. The asset panel uses it too.

Tests:
- ./check.sh: 150 files, 2226 tests, CHECK EXIT 0 (before 148/2214).
- 12 new unit tests: 6 for the reference rule, 4 for the link in
  createFieldRow, 2 for followsPress.
- 5 new e2e checks in 28l-ii: both menus, the link's look, click
  selects, double click opens.
- e2e 1-16,26,28,35,39,40 on the final build: ALL PASSED, 732 checks.
- Counter-probes: no "Open ..." entries, no underline and a second
  click never recognised each fail exactly at their checks.
- Check total 1341 (CI counted 1336 at fd6af84), computed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/editor/reference-link.test.ts

@@ -0,0 +1,89 @@
+/** The label of a reference row as a link to the asset it names (#221). */
+import type { FederatedPointerEvent } from 'pixi.js';
+import { describe, expect, it } from 'vitest';
+import { CreateAssetCommand } from '../../src/core/commands/assetCommands';
+import type { PropertyValue } from '../../src/core/model/Properties';
+import type { PropertyDef } from '../../src/core/properties/PropertySchema';
+import {
+  createFieldRow,
+  type FieldContext,
+  type FieldRow,
+  READ_ONLY,
+} from '../../src/editor/inspector/fieldFactory';
+import type { Label } from '../../src/ui';
+import { makeProject } from '../helpers';
+
+const SHADER: PropertyDef = {
+  key: 'shader',
+  label: 'Shader',
+  kind: 'asset',
+  default: '',
+  assetTypes: ['shader'],
+};
+
+interface Built {
+  row: FieldRow;
+  label: Label;
+  press(button?: number): void;
+  followed: string[];
+  shaderId: string;
+}
+
+/**
+ * Builds the row, runs the checks and destroys it before returning: widgets repaint in a microtask,
+ * and text cannot be measured without a document. The value `'shader'` stands for the shader's id.
+ */
+function withRow(value: PropertyValue, follows: boolean, checks: (b: Built) => void): void {
+  const fixture = makeProject();
+  const shader = new CreateAssetCommand('shader', 'Glow');
+  fixture.stack.execute(shader);
+  const followed: string[] = [];
+  const ctx: FieldContext = {
+    project: () => fixture.project,
+    ...(follows ? { followReference: (assetId: string) => void followed.push(assetId) } : {}),
+  };
+  const row = createFieldRow(ctx, SHADER, value === 'shader' ? shader.asset.id : value, READ_ONLY);
+  const label = row.row.children[0] as Label;
+  const press = (button = 0) => label.emit('pointerdown', { button } as FederatedPointerEvent);
+  try {
+    checks({ row, label, press, followed, shaderId: shader.asset.id });
+  } finally {
+    row.row.destroy({ children: true });
+  }
+}
+
+describe('a reference row label', () => {
+  it('is a link while the row names an asset, and a press follows it', () => {
+    withRow('shader', true, (b) => {
+      expect(b.label.underlined).toBe(true);
+      expect(b.label.eventMode).toBe('static');
+      b.press();
+      expect(b.followed).toEqual([b.shaderId]);
+    });
+  });
+
+  it('follows nothing on another button', () => {
+    withRow('shader', true, (b) => {
+      b.press(2);
+      expect(b.followed).toEqual([]);
+    });
+  });
+
+  it('is plain text while the row names nothing, and becomes a link once it does', () => {
+    withRow('', true, (b) => {
+      expect(b.label.underlined).toBe(false);
+      expect(b.label.eventMode).toBe('none');
+      b.row.setValue(b.shaderId);
+      expect(b.label.underlined).toBe(true);
+      b.row.setValue('gone');
+      expect(b.label.underlined).toBe(false);
+    });
+  });
+
+  it('stays plain text where nobody can follow it', () => {
+    withRow('shader', false, (b) => {
+      expect(b.label.underlined).toBe(false);
+      expect(b.label.eventMode).toBe('none');
+    });
+  });
+});